Traditional 401(k) Plan. A traditional 401(k) plan allows eligible employees (i.e., employees eligible to participate in the plan) to make pre-tax and/or after tax ROTH elective deferrals through payroll deductions. In addition, in a traditional 401(k) plan, employers have the option of making contributions on behalf of all participants, making matching contributions based on employees' elective deferrals, or both. These employer contributions can be subject to a vesting schedule which provides that an employee's right to employer contributions becomes nonforfeitable only after a period of time, or be immediately vested. Rules relating to traditional 401(k) plans require that contributions made under the plan meet specific nondiscrimination requirements. In order to ensure that the plan satisfies these requirements, the employer must perform annual tests, known as the Actual Deferral Percentage (ADP) and Actual Contribution Percentage (ACP) tests, to verify that deferred wages and employer matching contributions do not discriminate in favor of highly compensated employees.

Safe Harbor 401(k) Plan. A safe harbor 401(k) plan is similar to a traditional 401(k) plan, but, among other things, it must provide for employer contributions that are fully vested when made. These contributions may be employer matching contributions, limited to employees who defer, or employer contributions made on behalf of all eligible employees, regardless of whether they make elective deferrals. The safe harbor 401(k) plan is not subject to the complex annual nondiscrimination tests that apply to traditional 401(k) plans.

Safe harbor 401(k) plans that do not provide any additional contributions in a year are exempted from the top-heavy rules of section 416 of the Internal Revenue Code.

Employers sponsoring safe harbor 401(k) plans must satisfy certain notice requirements. The notice requirements are satisfied if each eligible employee for the plan year is given written notice of the employee's rights and obligations under the plan and the notice satisfies the content and timing requirements.

One-Participant 401(k) Plan. A one-participant 401(k) plan is sometimes called a: Solo-k or Solo-401(k), Uni-k or One-participant k. The one-participant 401(k) plan is not a new type of 401(k) plan. It is a traditional 401(k) plan covering a business owner with no employees, or that person and his or her spouse. These plans have the same rules and requirements as any other 401(k) plan.